The present invention relates to an apparatus for measuring surface stresses, and more particularly, to an apparatus for measuring surface stresses of glass coatings and transparent plastic products.
The term, glass coating, as used herein means enamelling given to the surface of copper instruments, enamelled surface of cast iron goods, enamel finishes for aluminum, brass, copper and other surfaces, and glazes for cloisonne, ceramics and procelain, all those products used mainly in daily life. The surface stress of these coatings plays significant roles in the quality and the decorative purposes of the products in the following points:
Firstly, it is necessary for these coatings to have an optimum compression stress in order for the coating to adhere well to the base and to have an excellent resistance against thermal and mechanical shocks as well as to corrosions by chemicals. If the surface stress is tensile, the cracks easily appear, the surface tends to peel off easily, and the resistance against chemicals becomes inferior. If the surface stress had an excessive compression force, the surfaces are more likely to present the phenomena generally referred to as "peeling" which is a voluntary removal of the surface coating film.
In the case of ceramics, on the other hand, the surface stress of the glaze is used intentionally as a tensile force thereby to cause hair cracks for decorative purposes. Since the designs and fine lines caused by such air cracks are determined depending on the tensile strength, selection of a suitable tensile strength is necessary depending on the purpose.
In the case of plastic products, stress is distributed through-out the entire product surface because of the thermal hysteresis at the time of molding, and non-uniform contraction accompanying the temperature changes after molding. If the stress is excessive, the product may become deformed, or cracks may appear after some years, and that may invite destructions and damages of the products. Solvent cracking phenomenon which is the appearance of cracks caused by the contact with catalysts may often occur.
It is, therefore, considered quite important for quality control that the surface stresses of glass coatings and plastic products by measured.
The present invention aims to provide an apparatus for a speedy and simple non-destructive measurement of the surface stresses of glass coatings and transparent plastic products.
The prior arts for measuring the surface stress of glass coatings and the like are particularly described in the following references:
H. Inada's "Fitness of Glaze and Body", Parts I and II, Interceram, No. 4, P. 397 (1978) and No. 1, P. 19 (1979) respectively describe, as the measurement method of the surface stress of glass coatings for ceramics, a method wherein a thin sample piece is obtained by vertically cutting the object being measured in the direction vertical to the coating surface and the photoelastic effect of the transmitted light is measured. However, this method is a destructive test requiring extra labor and costs, and the stress becomes alleviated in part as the thin test strip is cut out. When the method is applied to measuring the stress of the enamelled layer, the operation is generaly unsuccessful because the enamelled layer would become destroyed during the process of preparing the thin test strips.
Yogyo Kyokai Shi (Journal of Association of Ceramics, Japan), Vol. 72, No. 11-2, pp. 102-106 (1962) (in Japanese), describes a method of a surface stress measurement for enamelled surfaces wherein a thin plate is prepared from the material which is the same as the base to be coated, the enamel is flowed over one surface thereof, fired and then cooled to the room temperature, and the degree of camber of the thin plate appearing then is measured, thereby estimating the surface stress. This method does not measure directly the coating of the object being measured, but merely assumes the surface stress indirectly by measuring the sham test piece prepared separately. This method is not applicable to the case where the base plate is made of materials such as cast iron from which it is difficult to make a thin plate, and where the base plate is made of materials of which thermal expansion property is likely to change by the very small differences in the heat treatment conditions, such as cast iron, alloy steel, and non-ferrous alloys, since the measured values often tend to deviate from the actual surface stress values of the materials being measured.
Thus, neither one of the above-mentioned two measurement methods directly measures the coating surface of the object being measured; they are either the destructive test or the measurement test using sham test pieces and have the defects as above detailed, and therefore not satisfactory as a method of measuring the surface stress of glass coatings.
On the other hand, Acloque, P. and Guillemet, C., in Compt Rend 250 (1960) 4328 discloses a method of measuring the surface stress of thermally tempered glass. Their method utilizes the light propagated as an evanescent wave over the product surface, measuring the differences of photoelastic optical paths (sometimes referred to as retardation) as the function of propagation distance, and seeking the surface stress from the differential regarding the distance. Although this is a non-destructive measurement, it is defective in that exciting the evanescent wave is considered generally difficult, and since it is weak even when excited, it is extremely difficult to measure the differences in the optical paths as the function of the distance. It is further defective in that calculation used in obtaining the stress is complex and impractical.
As for the chemically tempered glass, the inventor of this invention published a paper dealing with a non-destructive method of measuring the surface stress in Yogyo Kyokai Shi 87, [3] 119 (1979) (in Japanese) by utilizing the property of the glass surface layer which has a high refractive index, and the light wave guide effect which propagates the light without scattering. However, the object to be measured for which the present invention is intended has no surface layers with a high refractive index and therefore the above method is not applicable.
Kitano proposed a method of seeking the surface stress of thermally tempered glass from measurement of the critical angle, in Yogyo Kyokai Shi 80, [4], 173, (1972) (in Japanese). This method again is not applicable to the case where the surface is not flat but curved or irregular, or where the surface layer is not uniform or has the light scattering property.
As mentioned above, the conventional technology of measuring the surface stress of glass coatings, transparent plastics, etc. did not obtain the satisfactory measurement values.